Sea Floor Spreading

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Sea Floor Spreading

• EQ What is the process of sea-floor spreading?

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Mid-Ocean Ridge

• Mid-Ocean Ridge The undersea mountain chain
  where new ocean floor is produced a divergent
  plate boundary

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Mid Ocean Ridge
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Sonar

• Sonar A device that determines the distance of
  an object under water by recording echoes of
  sound waves

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The sonar is used to map the ocean floor

• Sonar bounces sound waves off underwater objects
  and then records the echoes of these sound waves
• The time it takes for the echo to arrive
  indicates the distance to the object

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Evidence for Sea-Floor Spreading
In the 1960s, Harry Hess examined maps of the mid
ocean ridge. He proposed that the ocean floors
move like conveyor belts, carrying the continents
with them.
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Sea-Floor Spreading

• The process by which molten material adds new
  oceanic crust to the ocean floor

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What evidence did scientists find for sea-floor
spreading in the 1960s?

• Evidence from molten material
• Evidence from magnetic stripes
• Evidence from drilling samples

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Evidence From Molten Material

• Alvins crew found strange rocks shaped like
  pillows or like toothpaste squeezed from a tube
• Such rocks can form only when molten material
  hardens quickly after erupting under water
• The presence of these rocks showed that molten
  material has erupted again and again from cracks
  along the central valley of the mid-ocean ridge.
• http//www.pmel.noaa.gov/vents/nemo/explorer/conce
  pts/pillow_lava.html

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• Scientists discovered that the rock that makes up
  the ocean floor lies in a pattern of magnetized
  stripes
• 780,000 years ago, magnetic poles reversed
  themselves
• If they reversed today, the needle in a compass
  would point south instead of north
• The rock in the ocean is made of iron, which
  began as molten material

Evidence From Magnetic Stripes
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Evidence From Drilling Samples

• When scientists sampled the rocks, they found
  that the further away from the ridge the rocks
  were the older they were
• The younger rocks were always in the center of
  the ridges

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Subduction at Deep-Ocean Trenches
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Deep-Ocean Trenches

• A deep valley along the ocean floor through which
  oceanic crust slowly sinks towards the mantle

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Subduction

• Subduction The process by which oceanic crust
  sinks through a deep-ocean trench and back into
  the mantle a convergent plate boundary

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Guide For Reading What happens to the ocean
floor at deep ocean trenches?

• At deep-ocean trenches, two plates collide
  causing the denser of the two plates to dive back
  to the mantle. This process is known as
  subduction.
• Over tens of million of years, this material
  melts back into molten material and may rise
  again as new oceanic crust.

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Guide For Reading What is the process of
sea-floor spreading?

• At the mid-ocean ridge, molten material rises
  from the mantle and erupts. The molten material
  then spreads out, pushing older rock to both
  sides of the ridge.
• Over tens of millions of years, the process
  continues until the oldest ocean floor collides
  with the continental crust
• The more dense oceanic crust subducts (sinks)
  back into the mantle at a deep-ocean trench

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Subduction and Earths Oceans
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Subduction in the Pacific Ocean

• Subduction in the Pacific Ocean is occurring at a
  greater rate than sea-floor is expanding
• This is caused by the large amount of trenches

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Subduction in the Atlantic

• The Atlantic Ocean is expanding at a greater rate
  than subducting
• This is because of the low number of trenches in
  the Atlantic
• Over time the entire ocean gets larger and pushes
  against the continents

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Class Work and Homework

• Fill In
• What did I learn
• Confused
• Say
• Carefully read pages F33 F39, Answer questions
  1-5 on page F39